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Atomically thin Pt shells on Au nanoparticle cores: facile synthesis and efficientsynergetic catalysis

Engelbrekt, Christian; Seselj, Nedjeljko; Poreddy, Raju; Riisager, Anders; Ulstrup, Jens; Zhang,JingdongPublished in:Journal of Materials Chemistry A

Link to article, DOI:10.1039/c5ta08922k

Publication date:2016

Document VersionPublisher's PDF, also known as Version of record

Link back to DTU Orbit

Citation (APA):Engelbrekt, C., Seselj, N., Poreddy, R., Riisager, A., Ulstrup, J., & Zhang, J. (2016). Atomically thin Pt shells onAu nanoparticle cores: facile synthesis and efficient synergetic catalysis. Journal of Materials Chemistry A,2016(9), 3278-3286. https://doi.org/10.1039/c5ta08922k

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View Article OnlineView Journal | View Issue

Atomically thin P

aNanoChemistry, Department of Chemist

Kemitorvet 207, 2800, Kgs. Lyngby, DenmarbCentre for Catalysis and Sustainable Chem

University of Denmark, Kemitorvet 207, 280

† Electronic supplementary informa10.1039/c5ta08922k

Cite this: J. Mater. Chem. A, 2016, 4,3278

Received 4th November 2015Accepted 22nd January 2016

DOI: 10.1039/c5ta08922k

www.rsc.org/MaterialsA

3278 | J. Mater. Chem. A, 2016, 4, 327

t shells on Au nanoparticle cores:facile synthesis and efficient synergetic catalysis†

C. Engelbrekt,a N. Seselj,a R. Poreddy,b A. Riisager,b J. Ulstrupa and J. Zhang*a

We present a facile synthesis protocol for atomically thin platinum (Pt) shells on top of gold (Au)

nanoparticles (NPs) (Au@PtNPs) in one pot under mild conditions. The Au@PtNPs exhibited remarkable

stability (> 2 years) at room temperature. The synthesis, bimetallic nanostructures and catalytic properties

were thoroughly characterized by ultraviolet-visible light spectrophotometry, transmission electron

microscopy, nanoparticle tracking analysis and electrochemistry. The 8 � 2 nm Au@PtNPs contained

24 � 1 mol% Pt and 76 � 1 mol% Au corresponding to an atomically thin Pt shell. Electrochemical data

clearly show that the active surface is dominated by Pt with a specific surface area above 45 m2 per

gram of Pt. Interactions with the Au core increase the activity of the Pt shell by up to 55% and improve

catalytic selectivity compared to pure Pt. The Au@Pt NPs show exciting catalytic activity in

electrooxidation of sustainable fuels (i.e. formic acid, methanol and ethanol), and selective hydrogenation

of benzene derivatives. Especially high activity was achieved for formic acid oxidation, 549 mA (mgPt)�1

(at 0.6 V vs. SCE), which is 3.5 fold higher than a commercial < 5 nm PtNP catalyst. Excellent activity for

the direct production of g-valerolactone, an alternative biofuel/fuel additive, from levulinic acid and

methyl levulinate was finally demonstrated.

Introduction

Platinum (Pt) catalysts are crucial for a range of chemicalreactions in automotive, petroleum and energy conversionindustries.1 Pt has excellent catalytic properties but low avail-ability and high cost have shied focus from pure Pt toadvanced bimetallic Pt-based nanostructures.

Nanomaterial properties depend critically on chemicalcomposition, size, shape, crystallinity, and surface structure.Bimetallic nanostructures provide a strategy for design ofadvanced functional nanomaterials. A second element can, forexample, reduce the concentration of surface sites for catalyticpoisons – the so-called third-body effect.2 Metal monolayers alsohave dramatically altered properties by electronic interactionwith the underlying core.2 Facile synthesis of thin metal shellson top of a metal core offers attractive approaches to tuning thecatalytic activity as well as efficiently utilizing Pt.3 Gold (Au)nanostructures have been among the most studied systems dueto unique optical and electronic properties, and application ascatalysts for CO oxidation.4

Core–shell Au–Pt NPs show improved electrocatalysis,benetting from electrochemical stability of Au, high catalytic

ry, Technical University of Denmark,

k. E-mail: jz@kemi.dtu.dk

istry, Department of Chemistry, Technical

0, Kgs. Lyngby, Denmark

tion (ESI) available. See DOI:

8–3286

activity of Pt, and a performance boost from core–shell synergy.5

Au–Pt core–shell NPs can be prepared by precursor co-reductionfollowed by thermal treatment,6 by underpotential deposition(UPD) of copper (Cu) on Au followed by galvanic displacementby Pt precursors,7 or reduction via seed-mediated growth.8

Thermally driven methods require high temperature, strongreducing agents, controlled atmosphere, or non-aqueoussolvents.6 UPD is limited by the need for immobilization onconducting supports. UPD methods are superior for atomicallythin Pt shells, while wet syntheses oen provide thicknesses >1nm.8 Seed-based wet chemical syntheses offer potential forlarge-scale one-pot, green preparation, but the complexity of theprocesses gives poor yield and control of product morphology.Shell precursor reduction, diffusion through seed coatings, anddeposition on seed surfaces all have to be controlled.9

Non-toxic procedures and sustainability have emerged ascriteria in nanomaterial synthesis and applications.10 In ourprevious work, a starch and glucose based recipe, the saccha-ride-based approach to metal nanostructure synthesis(SAMENS), for the synthesis of gold nanoparticles (AuNPs)11 andother metal nanostructures was developed. A dynamics studyshowed that glucose acts as reducing agent and starch ascapping agent in phosphate buffer. The Good's buffer 2-(N-morpholino)ethanesulfonic acid (MES) participates in thereduction of HAuCl4 and adsorbs on the AuNP surface as well ascontrolling pH.12 Very small (1.6–1.8 nm) catalytically activestarch coated platinum nanoparticles (PtNPs) have also beenproduced by the SAMENS recipe.13 In the present study we have

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explored SAMENS for the synthesis of stable and uniform Au–Ptbimetallic nanostructures with high catalytic activity. UniformAuNPs serve as seeds for Au–Pt core–shell (Au@Pt) NPs withatomically thin Pt shells. Ultraviolet-visible light spectroscopy(UV-vis), NP tracking analysis (NTA), transmission electronmicroscopy (TEM) and electrochemical methods were employedto characterize the synthesis, structure, and properties. A cata-lyst of well dispersed Au@PtNPs with 12 wt% metal (3 wt% Pt,9 wt% Au) on graphitized carbon black (G-CB) was prepared andcomprehensively characterized. Starch capping was successfullyremoved thermally and the catalytic activity investigated byelectrocatalytic oxidation of small organic fuel molecules, andby selective aromatic ring hydrogenation (ARH) processes andhydrocyclization of levulinic acid (LA) and methyl levulinate(MLA).

Small organic molecules such as formic acid (FA), methanol(MeOH), and ethanol (EtOH), have been widely used for fuel cell(FC) systems as these fuels can be produced from renewablesources without harmful waste. FA is attractive since it is non-toxic and can be used at high concentrations due to low rates ofcrossing commonly used membranes. This leads to good FCperformance despite the low energy density compared to MeOHand EtOH. FA oxidation entails no C–C bond breaking which isa serious obstacle for EtOH oxidation. MeOH has receivedmuchattention as a C1 fuel with high energy density but faces chal-lenges of toxicity and membrane crossing. EtOH exhibits lowcrossover and as a potential 12-electron oxidation process hasa very high energy density.14 The Au@PtNP catalysts exhibithigh specic electrochemical Pt surface area (ECSA) and excel-lent catalytic activity for all the FA (FAOR), MeOH (MOR) andEtOH (EOR) oxidation reactions. Currents reach 549, 219 and229 mA (mgPt)

�1 for FA, MeOH and EtOH, respectively, in 0.1 Mfuel (scan rate 50 mV s�1), or 3.5, 2.0 and 3.1 times that of a <5 nm commercial PtNP catalyst (Sigma). Apart from improved Ptutilization in the atomically thin shell, synergy between the Ptshell and the Au core improves the Pt surface activity by asmuchas 55% relative to pure Pt.

ARH to cyclohexane derivatives is important in petrochem-ical and pharmaceutical industry.15,16 Chemoselective ARH ofbenzoic acid (BA) to cyclohexane carboxylic acid (CCA) is crucialin the synthesis of 3-caprolactam,17 but ARH requires harshconditions with a suitable catalyst to selectively hydrogenate thering. Noble metal catalysts (Ru–Pt,18 Rh/C,19 Pd/C,17,20 Pd–Ru21)have been extensively studied. Pd/C is traditionally used thoughthere are a number of limitations for ARH of BA to CCA, i.e. fastdeactivation, and need for high temperature (423 K) and highH2 pressure (15 MPa).22–24 Recently, Pt was reported as an activeARH catalyst with activity/selectivity depending strongly on NPshape and size.25

Catalytic conversion of cellulosic biomass to various plat-form molecules, e.g. g-valerolactone (GVL), is a key step inmethyltetrahydrofuran production. GVL can be obtained byhydrocyclization of LA, an inexpensive starting materialobtainable from biomass. Multiple catalytic strategies toproduce LA via biomass transformation are available,26 butefficient conversion of LA to GVL using green technology iswarranted. To date, numerous supported metal catalysts have

This journal is © The Royal Society of Chemistry 2016

been employed to hydrogenate LA to GVL with Ru-based cata-lysts being superior.27,28 Pt is active for LA hydrogenation, butactivity comparable to that of Ru using green technologies is yetto be obtained.

The Au@Pt catalyst introduced in this work efficiently andselectively catalyzes the ARH of BA and benzamide (BM), withhigh activity (TOF¼ 62 h�1) for GVL synthesis from both LA andMLA. To the best of our knowledge, ARH of BA and its deriva-tives, and catalytic production of GVL from LA/MLA over Au–Ptcatalysts has not been reported before.

Experimental sectionAu–Pt core–shell nanoparticles (Au@Pt NPs) with atomicallythin Pt shells

5–200 mL AuNP seed solution was prepared by pre-heatinga MES buffered mixture of glucose and starch for 5–10 min ina 95 �C water or oil bath, adding gold precursor, and allowing 30min for AuNP formation. The AuNP seed solution contained 2mM HAuCl4, 10 mM glucose, 0.6 wt% starch and 10 mM MES(pH 7). Following AuNP formation, the seed solution wasdiluted 4 times with water and 20 mM H2PtCl6 was added toobtain a Au : Pt mol ratio of 5 : 2 while heating was maintainedfor another 1–2 h. Complete reduction of the Pt precursor wasconrmed with UV-vis. The original seed solution had a deepred colour while the nal core–shell nanoparticle solution wasdark brown.

Electrocatalytic oxidation

Details about electrochemical measurements are given in thesupplementary information. Aer potential stabilization andCV in 0.1 M H2SO4, formic acid (FA), methanol (MeOH) orethanol (EtOH) were admitted into the cell to obtain a concen-tration of 0.1 M. All the experiments were performed at roomtemperature (r.t.).

Catalytic hydrogenation

A detailed description of hydrogenation experiments is givenin the supplementary information. In brief, chemoselectivearomatic hydrogenation of BA and BM, and cyclohydrogenationof LA and MLA was conducted in a 100 mL stainless steelautoclave reactor (Fig. S1†). Substrate, solvent, and catalyst weremixed in the autoclave, the reactor evacuated and pressurizedwith dihydrogen, heated to the target temperature, and main-tained for 0.5 to 24 h. Aer cooling to r.t., aliquots of the reac-tion mixture were submitted to gas chromatography (GC), andthe structure of the products identied using GC coupled withmass spectrometry (GC-MS).

Results and discussion

Controlling the size of the Au core allows tuning of the core–shellnanostructure properties. AuNPs in the range of 8–80 nm weresuccessfully prepared using a seeded growth approach inspiredby Bastus et al.29 The synthesis procedure and AuNP character-ization are provided in the supplementary information,

J. Mater. Chem. A, 2016, 4, 3278–3286 | 3279

Fig. 1 Size control of AuNP cores. 3D plots of (a) extinction, (b) core diameter and (c) hydrodynamic diameter of the 10 partitions. Plots forindividual partition are stacked along the y-axis with extinction (in a) and frequency (in b and c) representedwith the colour scheme from0 (black)to max (purple). The UV-vis spectra were normalized at the LSPRmax. The y-axis labels represent the size of the AuNPs derived from TEM images.UV-vis spectra and size distributions for the individual fractions are provided in Fig. S2 and S3.†

Fig. 2 (a) Principle of synthesis of Au@Pt NPs. AuNP cores (red) coatedby starch (green) are synthesized and the Pt shell (orange) of the Au@PtNPs subsequently formed in the same pot. (b) Extinction spectra of as-synthesized Au@Pt NPs (red), PtNPs (black) and AuNPs (blue) from theSAMENS method showing almost complete LSPR quenching by the Ptshell. (c) TEM micrograph of as-synthesized Au@Pt NPs. No evidenceof separate PtNPs was observed. Insert in (c) shows the size distribu-tion in nm of the NPs measured from TEM images. (d) HR-TEM of as-synthesized Au@Pt nanostructures showing no indication of a Pt shelldespite consistent Pt signal in EDX measurements.

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Fig. S2–S6 and Tables S1–S2.† Fractions were obtained at variousphases of the synthesis and characterized by UV-vis, TEM andNTA (Fig. 1a–c, respectively). A systematic increase in size wasconrmed with stable NP dispersions in the range of 8–80 nm.With increasing particle size, the localized surface plasmonresonance (LSPR) band initially narrowed followed by red-shi-ing and broadening. Above 80 nm the dispersions becameunstable leading to aggregation due to a thinning of the starchcapping with increasing size as seen by comparing TEM and NTAdata.

Synthesis of Au@Pt core–shell NPs

The two-step, one-pot synthesis strategy for the preparation ofAu@PtNPs from the SAMENS method is illustrated in Fig. 2a.The AuNP cores were rst prepared by chemical reduction ofHAuCl4 in buffered glucose–starch solution.12 Secondly, anatomically thin Pt shell was formed by reduction of H2PtCl6 toprovide Au@Pt core–shell NPs. The precursor was reduceddirectly aer the formation of AuNPs with the temperaturemaintained at 95 �C. The structures of the synthesis compo-nents are given in Scheme S1.† Formation of Au and Pt bime-tallic NPs requires control of precursor reduction. Too fastreduction of the Pt precursor leads to formation of separatePtNPs. Characteristic UV-vis spectra of the SAMENS preparedAu@PtNPs, PtNPs and AuNPs are shown in Fig. 2b. Theextinction from the Au@PtNPs (a continuum with a weakshoulder around 500 nm) resembles that of PtNPs with anindication of spectral features from AuNPs, indicating a drasticchange in the AuNP surface structure. No unreacted metalprecursors could be detected.

More details about the change in optical properties of theAu@PtNPs were obtained by varying the concentration of the Ptprecursor. 0.2, 0.5 and 1.0 mM Pt precursor (AuPt-0.2, -0.5 and-1.0, respectively) was employed while keeping the concentra-tion of Au xed at 0.5 mM. Fig. S7† shows UV-vis spectra of thethree samples aer terminated heating (solid lines) and 2 yearsof storage at r.t. (dotted lines). The lowest Pt precursorconcentration led to a dramatic broadening of the LSPR peak ofthe AuNP core strongly indicative of formation of a Pt shellaround the AuNPs. This damping by deposited Pt was previ-ously reported by Henglein when reducing Pt precursor withhydrogen gas.30 No signicant LSPR damping was observed with

3280 | J. Mater. Chem. A, 2016, 4, 3278–3286

higher Pt concentrations aer terminated heating or even aercomplete Pt precursor reduction. Under these conditions,separate PtNPs rather than core–shell NPs were formed and the

This journal is © The Royal Society of Chemistry 2016

Fig. 4 Elemental mapping of individual Au@PtNPs by STEM-EDX.High-resolution STEM images are shown in the left column, elementalmaps of Au and Pt in the middle columns, and a Au/Pt signal ratio mapin the right column (white/yellow¼ high Pt/Au, red¼ low Pt/Au). Scalebars correspond to 3 nm.

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colloids retained the characteristic red colour of the AuNPs,Fig. S8.† All NP dispersions remain stable aer more than 2years at r.t. owing to the efficient steric stabilization by thestarch coating. The LSPR peak of the AuNP core grew slightlyover time due to rearrangement of the Pt shell, Fig. S9.†

TEM was used to evaluate the size and elemental composi-tion of the Au@PtNPs, Fig. 2c and d. The size of the AuNP seedswas 8 � 2 nm, which was also found for Au@PtNPs. The size ofthe freshly formed AuNPs and AuNPs which had undergonecooling and reheating (to simulate the Pt shell formationprocess) was measured to 8 � 2 and 7 � 2 nm, respectively(Fig. S10†). In all cases, no signicant change in size wasidentied. Energy dispersive X-ray spectroscopy (EDX)conrmed that the Au@PtNPs contained both Au and Pt,Fig. 3a, at 76 � 1 and 24 � 1 mol%, respectively (based on 12EDX measurements).

This ratio was also found for Au@PtNP samples in which thePt shell was produced immediately aer AuNP formation. ThePt shell thickness was estimated based on ensemble EDXmeasurements assuming perfectly spherical 8 nm particles(as measured from TEM) with ideal core–shell geometry to 3 A.This was substantiated by individual NP EDX data providinga shell thickness of 2.9 � 0.7 A, Fig. 3b, which ts with a singlePt atom (2.77 A). In fact, scanning transmission electronmicroscopy with EDX (STEM-EDX) of single NPs shows evenlydistributed Pt and that Pt signals dominate around the NPperimeters, Fig. 4.

Further evidence of a Pt shell was found in the electro-chemical response of the bimetallic NPs. Cyclic voltammetry(CV) of the Au@PtNPs drop cast on glassy carbon electrodes

Fig. 3 Identification of the Pt shell. (a) EDX spectrum of as-synthesized Aimages of individual Au@Pt NPs. Red band shows mean � 1 standard devas-synthesized Au@Pt NPs, single-crystal Pt and Au, (d) CVs of as-synthesCE ¼ Pt, scan rate ¼ 50 mV s�1). Pt and Au oxide reduction and hydrog

This journal is © The Royal Society of Chemistry 2016

revealed broad anodic signals from 0.6 to 1.3 V from oxidationof the Pt shell and uncovered areas of the Au core, and clearcathodic peaks corresponding to the reduction of PtOx (0.36 V)and AuOx (0.86 V) showing that both metals are present at thesurface, Fig. 3c. The oxide reductions in the bimetallic NPsaccord with observations for pure SAMENS Au and Pt NPs testedunder the same conditions (Fig. 3d) though the reduction

u@Pt NPs. (b) Estimated Pt shell thickness based on EDX and HR-TEMiation and the green dashed line the diameter of metallic Pt. CVs of (c)izedmonometallic NPs and bare glassy carbon (0.1 MH2SO4, RE¼ RHE,en adsorption/desorption are highlighted.

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potentials are shied slightly, Table S3.† The Au oxide reduc-tion is shied 40 mV negatively suggesting that Au oxide isstabilized by the Pt shell whereas the Pt oxide reduction isshied 30 mV positively. The hydrogen adsorption/desorptionregion at low potentials is characteristic of Pt and furthersupports the abundance of exposed metallic Pt.

Fig. 5 TEM micrographs of (a) as-synthesized Au@Pt/G-CB catalystand (b) Au@Pt/G-CB catalyst after thermal treatment to remove thestarch coating.

Catalyst preparation and characterization

The Au@PtNPs were immobilized on a graphitized carbon blacksupport (G-CB), which is robust against degradation due toa high content of sp2 carbon.31 Good wetting of the support wasensured by ultrasonication, prolonged stirring and heating inwater prior to mixing with as-synthesized Au@PtNPs. The G-CBsuspension was added to the Au@PtNP colloid shortly aer Ptshell formation, and adsorption completed within 1 h,Fig. S11.† The starch coating used in this work was not cova-lently bound to the NP surface and could be removed viaoxidation by heating in air at 300 �C for 1 h. Fig. S12a† showsthermogravimetric analysis (TGA) conrming that starch begandecomposing/oxidizing around 200 �C, while the bare G-CBsupport was stable up to 700 �C. TGA of the Au@Pt/G-CB beforeand aer thermal treatment are identical except for the low-temperature region corresponding to mass loss from starch andno indication of starch was found aer thermal treatment.Oxidation of the carbon support was catalyzed by theAu@PtNPs. The onset of oxidation thus shied from 730 to570 �C in the presence of the bimetallic NPs. The oxidation ofthe starch capping was also catalyzed by the Au@PtNPs asindicated by shis of the two exothermic peaks in the differ-ential thermal analysis (DTA) of starch from 300/390 �C to190/260 �C, Fig. S12b.† The masses nally obtained wereassigned to the adsorbed Au@PtNPs and corresponded to 12–13wt% of the catalyst (excluding starch).

X-ray photoelectron spectroscopy (XPS) based on the Au (93–81 eV) and Pt (80–68 eV) 4f signals provided 67 � 3 wt% ofAu@Pt/G-CB, Fig. S13,† with Pt comprising 21.8 � 0.7 wt% andAu 45 � 3 wt%. More Pt was found with XPS, a surface specictechnique, than EDX due to Pt-rich NP surface. The thermaltreatment did not affect the Au and Pt 4f spectra, but theshoulder in the C 1s spectrum at 286.5 eV corresponding to C–Oand the O 1s signal disappeared aer treatment. X-ray powderdiffraction (XRD) and TEM were used to verify that the thermaltreatment did not compromise the bimetallic NP nanostructureas no change in the diffraction pattern was found, Fig. S14.†Evaluation of the crystallite size using the Scherrer equation onthe metal peaks at 38 and 65� conrmed that no line narrowingand crystallite growth had occurred. TEM further supportedthat the overall catalyst structure was stable during the starchremoval process, Fig. 5, though slight agglomeration of the NPswas observed.

Further electrochemical analysis of the Au@Pt/G-CB surfacewas undertaken, Fig. 6a. The CVs of bimetallic catalysts matchthat of immobilized, unsupported Au@Pt NPs but provide sixtimes higher currents attributed to the optimized dispersion inthree dimensions on the G-CB. The commercial catalyst exhibitscharacteristic Pt response throughout the applied potential

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range. The well-dened features of the supported catalysts allowfor deeper evaluation of their electrochemical properties. Theelectrochemically active surface areas (ECSAs) of the Au@Pt/G-CB catalysts and a commercial Pt/G-CB catalyst were evalu-ated by integrating the charge in the hydrogen adsorption/desorption region (H-UPD) for Pt and the metal oxide reductionpeaks for Pt and Au, Table 1 and Fig. S15.† The specic ECSA ofPt in the commercial catalyst was 21.2 and 21.8 m2 g�1 fromH-UPD and PtOx, respectively, which is consistent with previousreports.32 The specic ECSA of Pt in Au@Pt/G-CP was as high as45.3–52.5 m2 g�1, 2.1–2.4 times that of the commercial catalystdepending on whether hydrogen adsorption or PtOx reductionis considered, Table 1. The Pt ECSA decreased drastically withthermal treatment (39% from H-UPD and 33% from PtOx)which was unexpected from the XRD and TEM results. Indeed,the Au ECSA dropped by only 10% suggesting that agglomera-tion is not the only effect of heating. It is likely that the Pt shellrearranges forming thicker areas during heating. Consideringthe ECSAs from metal oxide reduction, the Pt coverage can beestimated by ECSAPt/ECSAPt+Au. The Pt coverage decreases from73 to 66% during thermal treatment due to contraction of the Ptshell. Braidy et al. showed with detailed EDX mapping how Au–Pt core–shell nanoparticles separate into pure phases joined bya at interface at 600 �C in inert atmosphere.33 They reportedthat the core–shell structure is stable at 300 �C. The smaller NP,thinner Pt shell, air atmosphere and catalysed starch oxidationin the current system could thus facilitate shell rearrangementat lower temperatures, e.g. 300 �C. The increase in the Au : Ptratio aer thermal treatment observed in XPS further supportsthat more Au is found at the particle surface. The greaterreduction in Pt ECSA relative to Au ECSA could also be caused bypartial dissolution of the Pt shell in the bulk of the NP. Both as-synthesized and thermally treated Au@Pt/G-CB catalysts werefurther tested in catalytic systems.

Electrocatalysis

The Au@PtNP performance in FC anode electrocatalysis wasevaluated by studying electrooxidation of small organic mole-cules used in direct alcohol/carboxylic acid FCs. The threetested systems vary in complexity from simple 2-electronoxidation of FA to the oxidation of EtOH in a 12-electron

This journal is © The Royal Society of Chemistry 2016

Fig. 6 Cyclic voltammograms of Au@Pt/G-CB (red), t-Au@Pt/G-CB where starch has been removed (blue) and commercial Pt/G-CB (black) in(a) pure electrolyte (0.1 M H2SO4), or electrolyte with (b) 0.1 M formic acid, (c) 0.1 M methanol or (d) 0.1 M ethanol. In all cases RE¼ RHE, CE¼ Ptand scan rate ¼ 50 mV s�1.

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process. Fig. 6b–d shows CVs recorded at 50 mV s�1 in pure 0.1MH2SO4 electrolyte with 0.1 M FA, MeOH or EtOH, respectively,Table 1. As-synthesized Au@Pt/G-CB, thermally treated Au@Pt/G-CB without starch (t-Au@Pt/G-CB) and commercial PtNPs(Sigma-Aldrich) on G-CB (Pt/G-CB) were tested and are pre-sented in Fig. 6 as red, blue and black lines, respectively. Alldata were recorded at identical Pt loadings and are shown as Ptmass specic activities.

Formic acid (FA) electrooxidation. Electrocatalytic oxidationof FA to CO2 on a metal surface may proceed via a triple pathmechanism though the exact mechanism is under debate.2

Apart from direct decomposition of FA to CO2, oxidation canoccur through dehydrogenation and adsorption of formate, ordehydration and subsequent oxidation of strongly adsorbedCO.34 CVs of electrochemical oxidation of FA are shown inFig. 6b and display two anodic peaks at 0.30 and 0.61 V vs. SCEcorresponding to oxidation of FA via adsorbed formate and CO,respectively.32 The peak ratio, the pathway factor, indicates thecatalyst selectivity towards direct oxidation through non-poisoning dehydrogenation. The bimetallic catalysts exhibithigher pathway factors than the commercial PtNP catalyst andimproved selectivity towards the formate pathway relative topure Pt. The largest factor was observed for t-Au@Pt/G-CB morethan 2.5 times that of Pt/G-CB. The mass activities estimatedfrom the peak currents around 0.6 V were 556, 315 and 158 mA(mgPt)

�1 for Au@Pt/G-CB, t-Au@Pt/G-CB and Pt/G-CB, respec-tively, Table 1. The mass activity of the Au@Pt/G-CB is improved3.5 times compared to the commercial catalyst and is higherthan or comparable to several recently reported Pt-based cata-lysts such as Pt nanotetrahedra,35 PtCu trigonal bipyramid

Table 1 Electrochemical characterization and fuel oxidation efficiency o

Catalyst

ECSA [m2 g�1]

Coverage GPta

Mass activity [mA

FAOR, 0.61 V MH-UPD PtOx AuOx

Au@Pt/G-CB 45.3 52.5 6.50 73% 556 2t-Au@Pt/G-CB 27.5 35.1 5.85 66% 315 1Pt/G-CB 21.2 21.8 — (100%) 158 1

a Pt surface area relative to total metal surface area based on metal oxide r

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nanoframes36 and PtPb nanoowers,32 which were even ob-tained at higher FA concentrations (0.25–1 M) than thoseemployed here. With a higher specic surface area of thebimetallic structures than of pure PtNPs, a higher mass activityis expected. Normalizing the CVs to Pt ECSA discloses a syner-getic contribution from the Au core, Table 1, and a surface 55%more active on the Au@Pt/G-CB compared to the Pt/G-CB. Thesurface of the thermally treated bimetallic catalyst was slightlyless active (1.02 mA cm�2) than that of the as-synthesized one(1.14 mA cm�2) but still higher than for pure Pt (0.74 mA cm�2).The Au core thus improves the Pt utilization not only byincreasing active surface area but also by modifying the activityitself of that surface.

Methanol (MeOH) electrooxidation. The electrochemicaloxidation of MeOH on Pt involves several intermediate steps, i.e.dehydrogenation, CO-like species chemisorption, OH� (or H2O)adsorption, chemical interaction between adsorbed CO and–OH compounds, and CO2 evolution. The primary by-productsin methanol oxidation are formaldehyde and FA.37 Electro-oxidation of MeOH is a 6-electron process which gives rise to theformation of strongly adsorbed CO species in linear or bridge-bonded form, i.e. catalyst poisoning.37 The electrocatalyticactivity of Pt can be promoted by the presence of a second metal(e.g. Ru, Sn, Au)38 acting either as an adatom or a bimetal. CVs inFig. 6c show the electrooxidation of MeOH on the three testedcatalysts. One main anodic peak is observed at 0.54 V in theforward scan and represents oxidation of MeOH throughdehydrogenation and adsorption of residues.39 Again, the as-synthesized Au@Pt/G-CB demonstrated the highest mass-specic peak-current density of 219mA (mgPt)

�1, or twice that of

f Pt-based catalysts

mgPt�1] Surface area activityb [mA cm�2]

OR, 0.54 V EOR, 0.62 V FAOR, 0.61 V MOR, 0.54 V EOR, 0.62 V

19 229 1.14 0.45 0.4797 162 1.02 0.64 0.5310 74 0.74 0.51 0.34

eduction. b Data normalized to the average ECSA from H-UPD and PtOx.

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the commercial Pt catalyst (110 mA (mgPt)�1). This value is

higher than obtained previously at 5-times higher fuel concen-tration with Au–Pt core–shell systems.40 Recently, currentsaround 500 mA (mgPt)

�1 in 1 M MeOH with Au–Pt core–shellNPs on carbon support,41 PtRu NPs on N-doped graphene42

and Pt NPs on TiO2/N-doped carbon nanocomposite wereobtained.43 Peak-currents increase with increasing MeOHconcentration.44 In contrast to FA oxidation, the thermallytreated catalyst showed only slight reduction in activity (197 mA(mgPt)

�1) indicating that reduction of ECSA induced by heatingwas counteracted by increased activity towards the MOR.Indeed, the surface activity was highest for t-Au@Pt/G-CB at0.64 mA cm�2, or 25% higher than for pure Pt. The surface ofthe as-synthesized Au@Pt/G-CB was less active for MOR thanthe commercial Pt catalyst. These observations further supportthat the chemistry of the Pt shell is modied by interaction withthe Au core affecting the MeOH electrooxidation.

Ethanol (EtOH) electrooxidation. EtOH electrooxidationleads to several products, acetic acid, acetaldehyde, acetylradical, CO and CO2. Its complete oxidation delivers 12 elec-trons per EtOH molecule giving very high energy density.14

However, breaking C–C bonds is difficult and CO2 is onlyproduced to a small extent in electrochemical oxidation.14

Several spectroscopic and electrochemical techniques havebeen applied to elucidate the mechanism of EtOH electro-oxidation on Pt-based materials.45,46 The contributions of thedifferent pathways depend strongly on EtOH concentration.47

Above 0.1 M, less water is present at the surface and formationof CO2 and acetic acid is signicantly inhibited, while produc-tion of acetaldehyde starts to increase.47

The activity of the three catalysts towards the EOR is illus-trated by the CVs in Fig. 6d showing a main oxidation peak at

Fig. 7 (a) Conversion (green), yield (black) and TOF (red) for catalytic hydcatalyst NP structures are shown on top of the columns. (b) Reaction sch(I, II, and V) as-synthesized Au@Pt/G-CB, (III, IV, and VI) thermally trea0.32 mol% Pt, (V–VII) 1.5 mol% Pt. Solvent: (I, III, V–VII) water, (II and IV)

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0.62 V and a second peak at 1.0 V which is due to furtheroxidation of intermediate products.47 The catalyst activitiesfollow the same trend as for the FAOR with mass-specic peakcurrents of Au@Pt/G-CB (229 mA (mgPt)

�1) and t-Au@Pt/G-CB(162 mA (mgPt)

�1) being 3.1 and 2.2 times higher than forcommercial Pt/G-CB (74 mA (mgPt)

�1). As with the MOR, the PtECSA-specic activity was highest for the thermally treatedcatalyst, t-Au@Pt/G-CB followed by the as-synthesized Au@Pt/G-CB being 55% and 38% higher than Pt/G-CB, respectively(Table 1). Notably the ratio between the 0.62 and 1.0 V peaks forAu@Pt catalysts was opposite that of Pt/G-CB indicating modi-cation of surface specicity for EtOH oxidation in the presenceof the Au core. This is supported by a recent theoretical study inwhich EtOH and acetyl radical were found to exhibit closerbinding interactions on Pt/Au(111) than on pure Pt(111).48

Catalytic hydrogenation of benzenes

The activity of Au@Pt NPs towards hydrogenation (reduction)reactions was probed using substituted benzenes and tested inthe hydrocyclization of LA to GVL, Scheme S2.† ARH of BM tocyclohexane carboxamide (CCM) was chosen as a standardreaction to optimize the reaction parameters, such as solventeffects and thermal pre-treatment, before testing BA, andLA/MLA hydrogenation. The Au@Pt/G-CB was screened in its as-synthesized and thermally treated forms using both protic(water) and aprotic (toluene) solvents to study how the solventenvironment affects the catalyst activity. Fig. 7 (and Table S4†)shows the catalysts activity for chemoselective hydrogenation ofBM to CCM in both water and toluene using H2 as reducingagent. The Au@Pt catalyst was active and highly selective for BMhydrogenation under all reaction conditions. The thermallytreated catalyst (Fig. 7III) showed no superior activity over the

rogenation of 0.5 mmol BM (I to IV), 0.5 mol MLA (V to VII). Schemes ofemes and general conditions for BM and MLA hydrogenation. Catalyst:ted Au@Pt/G-CB, (VII) commercial Pt/G-CB. Catalyst loading: (I–IV)toluene. Reaction time: (I–IV) 24 h, (V–VII) 30 min.

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as-synthesized catalyst in water (I). Instead, a slight decrease(from 78 to 71% CCM yield) was noticed, most likely due to thenanostructural changes induced by the thermal treatment, asdiscussed previously. The activity of as-synthesized Au@Pt/G-CB dropped to 28% when going from water to toluene (II), butcould be recovered to nearly that of water by removing starch(IV). This is due to the low solubility of starch in toluene,Fig. S16.† The non-hydrated starch molecules on the Pt surfacein non-polar environment do not extend into the solution butrather collapse resulting in limited access to the active sites. Thehydrogenation rate can be related to H-bond donation of theenvironment around the active site. A lower activation energybarrier for protic solvents, due to strong interaction betweenprotic solvents and substrate by hydrogen bonding, leads tohigher hydrogenation rates.49 This is of minor importance inthe present system where the activity of thermally treatedcatalyst is similar in both solvents (III and IV). Finally, it isworth noting that no hydrogenation product of toluene wasobserved.

The Au@Pt/G-CB catalyst showed also excellent catalyticactivity for chemoselective hydrogenation of BA under mildconditions. Quantitative yields (100%) of CCA were achievedwith > 99% selectivity in 24 h at 1 atm H2 pressure and 80 �C.N-doped carbon supported Pd nanoparticles was recentlyshown to bring a nine-fold increase in activity compared tocommercial Pd on activated carbon for the hydrogenation of BAunder similar conditions.49 The Au@Pt/G-CB introduced in thiswork provided as much as 15-fold increased turn-over frequency(TOF) at 0.3 mol% Pt with Au (1.0 mol%). No traces of BAhydrogenation to benzyl alcohol were observed under the givenreaction conditions and CCA was the only product.

Hydrocyclization of LA to GVL

Carbon supported Pt catalysts have previously shown pooractivity and over-hydrogenation of GVL resulting in low selec-tivity.27,50 Improved activity was reported by Kon et al. with Pt/Cresulting in 14% yield of GVL in 1 h at relatively low pressure(8 bars) and high temperature (200 �C).51 Au@Pt/G-CB wasscreened for LA hydrogenation followed by cyclization to GVL.Quantitative yields of GVL were attained with excellent selec-tivity (> 99%) (Table S5, VIII†). The commercial Pt/G-CB catalystwas applied as a reference providing similar results. To deter-mine the TOF, shorter reaction times were chosen. Reactionwith commercial Pt/G-CB for 5 h gave quantitative yields of GVLat a TOF of 13 h�1 (Table S5, IX†). The activities of Au@Pt NPand commercial Pt catalysts (Fig. 7V–VII) were compared at 30min reaction times. Commercial Pt/G-CB catalyst yielded 40%more GVL than as-synthesized Au@Pt/G-CB but the highestyields were obtained with thermally treated Au@Pt/G-CB. Theremoval of the starch capping thus resulted in a signicantlyhigher catalytic activity than that of commercial Pt/G-CB (48%yield vs. 42%) at a TOF of 62 h�1. This observation highlightsthe need for starch removal for LA hydrocyclization. The largermolecular size of LA relative to other tested substrates (FA,MeOH, EtOH, BA, BM) may hinder diffusion through the starchlayer and limit the activity. The surface structure changes

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induced by the heating might also increase the activity towardsLA hydrocyclization. All the catalysts showed excellent selec-tivity to GVL with no detectable over-hydrogenation (formationof valeric acid, 2-Me-THF).

Conclusions

The green synthesis protocol SAMENS was developed forsynthesis of highly active starch protected 8 � 2 nm core–shellNPs (Au@PtNPs) consisting of a spherical Au core and a 3 Aatomically thin Pt shell. The G-CB supported Au@Pt catalyst(3 wt% Pt, 9 wt% Au) was thoroughly characterized. Starchremoval by heating changed the chemistry of the Pt shell as seenin the electrochemical characterization, and strongly affectedthe catalytic performance in electrochemical oxidation andhydrogenation, which possibly relate to structural changes ofthe Pt shell. Catalytic activities for several electrooxidation andhydrogenation reactions were investigated and compared tocommercial Pt/G-CB. The high specic Pt surface area of thethin Pt shell led to superior activity of Au@Pt/G-CB over Pt/G-CBfor electrochemical oxidation of important FC fuels (FA, MeOHand EtOH). Au@Pt/G-CB mass activities (in mA (mgPt)

�1) of 556for FAOR, 219 for MOR and 229 for EOR were 3.5, 2.0 and 3.1times higher than for the commercial Pt/G-CB catalyst. Thecore–shell structure gave synergy between the Pt shell and theAu core, as well as limiting inactive Pt. The activity of the Ptsurface increased with up to 55%.

The presence of the starch coating had insignicant effectson the catalytic activity for ARH of BM in water. Removal ofstarch prompted a 2.5-fold increase in activity in toluene inwhich the hydrophilic starch chains collapse, passivating thesurface. The Au@Pt/G-CB catalyst was active for GVL productionfrom LA/MLA via hydrogenation/cyclization. For this system,the highest activity (TOF ¼ 62 h�1) was exhibited by the ther-mally treated catalyst.

The facile, green and robust synthesis presented hereproduces highly active and well-dened bimetallic nano-structures and offers prospects of further tuning catalyticactivity/selectivity by the Au core size, Pt shell thickness andthermal treatment.

Acknowledgements

Assistance with STEM-EDX experiments from WilhelmusHuyzer (DTU CEN), TGA measurements from Larisa Seerup(DTU Energy Conversion and Storage) and the experimentalhydrogenation setup from Eduardo J. Garcıa-Suarez (DTUChemistry) are appreciated. Financial support from DanishCouncil for Independent Research Technology and ProductionSciences (DFF-1335-00330) and the Lundbeck Foundation(R141-2013-13273) are acknowledged.

Notes and references

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